This invention relates to an optical waveguide and a display device. In particular it is relevant to display devices in which image bearing light is injected into a waveguide, is expanded in two orthogonal dimensions to form a visible image and is released from the waveguide.
A prior art device of this type is shown in U.S. Pat. No. 6,509,529. In that device a slab waveguide contains three separate differently-oriented diffraction gratings, one to inject the light into the waveguide, a second to expand it in a first dimension, and a third to expand it in a second dimension and to release the thereby-formed image from the waveguide. Whilst this arrangement is simpler than earlier proposals in which the third grating is in a separate waveguide from the first two, it still poses manufacturing challenges in that the three differently-oriented gratings (whether produced holographically or a surface-relief gratings) still must be very accurately aligned relative to each other if good results are to be achieved.
The present invention seeks to alleviate these difficulties by offering a solution which can be implemented using fewer diffraction gratings.